P4 MAGNETISM AND MAGNETIC FIELDS

Question 1

in which direction do magnetic field lines point?

Answer 1

north to south

Question 2

what’s the difference between a permanent and induced magnet?

Answer 2

a permanent magnet produces its own magnetic field whereas an induced magnet is a material which becomes a magnet when placed in a magnetic field

Question 3

what are the characteristics of the magnetic field of a magnet?

Answer 3

lines never intersect, fields density indicates strength of magnetic field

Question 4

how does the behaviour of a magnetic compass show that the core of earth is magnetic?

Answer 4

when a plotting compass is placed in earth’s magnetic field, the north pole of the compass will line up with earth’s magnetic field lines and point to magnetic south

Question 5

how can you show that a current can create a magnetic effect?

Answer 5

if you place a magnetic compass in that region, the magnetic field can deflect its needle

Question 6

how can you describe the directions of the magnetic field around a conducting wire?

Answer 6

using the right hand grip rule: the fingers curl in the direction of the magnetic field and the thumb points in the direction of the current

Question 7

what does the strength of the field depend on?

Answer 7

current and distance from the conductor

Question 8

how can solenoid arrangements enhance the magnetic effect?

Answer 8

increase the number of turns on the coil, increasing the current, placing an iron core inside the solenoid

Question 9

how do a magnet and current carrying conductor exert a force on one another?

Answer 9

a wire carrying a current creates a magnetic field which interacts with the magnet’s magnetic field, producing a force (motor effect)

Question 10

what does the thumb, forefinger and middle finger represent in Fleming’s LHR?

Answer 10

force, field, current

Question 11

how does the force exerted from a magnet and a conductor cause rotation in electric motors?

Answer 11

the forces acting on a coil of wire in a magnetic field cause it to rotate

Question 12

how do electric motors work?

Answer 12

electric motors are made of a coil of wire in between 2 permanent magnets. dc runs through the wire and experiences a force (motor effect) exerted by the magnetic field. for every half turn of the coil, the split ring commutator causes the current to switch direction, ensuring a turning effect is experienced for every half turn so the motor keeps turning.

Question 13

how does a split ring commutator work?

Answer 13

the split in it when attached to the power supply reverses the current flow when the split reaches a vertical/open position in the circuit

Question 14

what can a change in the magnetic field around a conductor lead to?

Answer 14

an induced pd across its ends, which could drive a current, generating a magnetic field that opposes the original change

Question 15

how is the generator effect used in an alternator to generate ac?

Answer 15

a coil rotates in a magnetic field. as one side of the coil moves up through the magnetic field, a pd is induced in 1 direction. as the rotation continues and that side of the coil moves down, the induced pd reverses direction. this means the alternator produces a current that’s constantly changing

Question 16

how does the generator effect produce dc in a dynamo?

Answer 16

a coil rotates inside a magnetic field which generates the current. the split ring commutator changes the connection every 1/2 turn to keep a current flowing in the same direction

Question 17

how is the effect of an ac used in transformers?

Answer 17

a pd drives an ac through the primary coil. the primary coil current produces a magnetic field, which changes as the current changes. the iron core increases the strength of the magnetic field. the changing magnetic field induces a changing pd in the secondary coil. the induced pd produces an ac in the circuit.

Question 18

how does the ratio of potential differences across the 2 coils in a transformer depend on the ratio of the number of turns in each?

Answer 18

the higher the number of turns on the coil, the higher the pd in that coil

Question 19

how does the microphone work?

Answer 19

pressure variations in sound waves cause the flexible diaphragm to vibrate. the vibrations of the diaphragm cause vibrations in the coil. a pd is induced, causing a current to flow. the changing size and vibration of the induced current matches the vibrations of the coil. the electrical signals generated match the pressure variations in the sound wave

Question 20

how does a loudspeaker work?

Answer 20

a current in the coil generates a magnetic field. the magnetic field interacts with the permanent magnet’s field, generating a force and pushing the cone outwards. a current is made to flow in the opposite direction, the direction of the magnetic field reverses and the force on the cone now pulls it back in. the repeated alternating of the current direction makes the cone vibrate in and out. the cone vibrations cause pressure variations in the air (sound waves)